1. Field of the Invention
This invention relates to an imaging lens suitable, for example, as a photographic lens which uses a medium in which exists a so-called refractive index distribution in which the refractive index thereof gradually varies about the optic axis in a plane orthogonal to the optic axis.
2. Description of the Prior Art
Heretofore, almost all imaging lenses have been constructed by the use of a medium whose refractive index is uniform, and for example, an imaging lens having a half angle of view of 23.degree. and F-No. of the order of 1.4-1.8 is usually constructed of six to seven lenses of a homogeneous medium, as noted in a photographic lens. If such lens can be constructed of fewer lenses, it will be very advantageous in respect of a reduction in the labor required for lens working, simplification of the lens holding mechanism and a reduction in the possibility of injecting a fabrication error such as eccentricity into the manufacture of the lens. However, according to the designing technique using the conventional homogeneous medium, it is very difficult to reduce the number of constituent lenses while maintaining the optical performance.
On the other hand, in recent years, the distributed index lens has been drawing attention as a novel optical technique, and numerous applications thereof to an array lens used an erect one-to-one magnification imaging element, a collimator lens taking only the imaging on or near the axis into consideration, a pickup lens for optical disc, etc. have been proposed. Further, in the article of Atkinson et al. (Applied Optics, Vol.21, No.6, 1982), application of the distributed index lens to a photographic lens is reported. This photographic lens is comprised of two distributed index lenses, that is, on the object side, there is disposed a first lens having a meniscus shape in which the thickness thereof is smallest on the optic axis and increases away from the optic axis and having its convex surface facing the object side and having a refractive index distribution in which the refractive index thereof is highest on the optic axis and continuously decreases away from the optic axis, and on the image side, there is disposed a second lens having a meniscus shape in which the thickness thereof is smallest on the optic axis and increases away from the optic axis and having its convex surface facing the image side and having a refractive index distribution in which the refractive index thereof is highest on the optic axis and continuously decreases away from the optic axis. By this construction, a standard lens in which the F-No. is 2 is provided and, as compared with the conventional imaging lens comprising homogeneous lenses, this lens greatly reduces the number of lenses used. An application of the distributed index lens to an imaging lens having a relatively great angle of view is hardly found except in this report.